A method of electroplating surfaces with tin that uses an electrolyte containing a phenolsulfonic acid and an addition agent of the ethoxylated naphthol sulfonic acid type is known and employed under the trade mark of “ferrostan”.
The method includes steps of electrolytic degreasing (cleaning), electrolytic pickling, electrolytic tinning using the electrolyte containing (g/l):
Sn2+28-34Free phenolsulfonic acid14-17(in conversion to sulfuric acid)“ENSA-6” agent3-6reflowing, electrolytic passivation and oiling of tinplate.
The “ferrostan” technology provides for producing tin coating over a rather wide range of basic process parameters, but the presence of large quantities of phenol- and naphthol-containing products in the composition makes the process environmentally inappropriate.
A method for electrolytic tinning described in U.S. Pat. No. 6,217,738 provides the use of one or more acids obtained by modifying a sulfur acid (phenolsulfonic, toluenesulfonic, sulfamic, alkyl sulfonic etc.) and one or more addition agents that include di- and tri-substituted phenols with substituents comprising a secondary, tertiary or quaternary nitrogen atom. The disadvantages of the above method are the use of environmentally inappropriate products and complicated composition which in many cases includes a mixture of two acids and two addition agents making the application of this process in commercial tinplating lines highly difficult.
Another method for electrolytic tinning is also known and described in R.F. Pat. No SU1678094. This method provides the use of a sulfamic acid based electrolyte further containing polyethylene glycol sulfate and sulfosalicylic acid as addition agents. Extremely narrow limits of the process parameter variations and, especially, the electrolyte temperature make this process unsuitable for practical implementation. Moreover, the presence of the sulfosalicylic acid in the composition has bad effects on the environment that are an obstacle to its implementation.
Another method for electrolytically tinned plate production is disclosed in R.F. Pat. No RU2103418. Such document describes a method for coating a metal strip by using an electrolyte containing tin in form of bivalent ions, sulfamic acid and a nitrogen-bearing block copolymer of ethylene oxide. The current passes through the strip at densities of 20 to 70 A/dm2. The electrolyte composition (g/l) is as follows:
Tin in a form of bivalent ions20-37Sulfamic acid (total)100-140“Proxamin-385”0.5-2.5Waterthe restwherein the coating is applied at 20-70° C. and “proxamin-385” is used as the nitrogen-containing block copolymer of ethylene and propylene oxides.
The above process is distinguished by the environmental appropriateness since it doesn't provide the use of any high-hazard materials and is implemented within the wide range of process parameters (temperature, current density). The electrolyte possesses a high dissipating ability. But this electrolyte provides a high quality of coating only at current densities higher than 20 A/dm2, while when producing electrolytic tinplate, the most-used, with coating weight of 1.0 to 2.0 g/m2 per side, the required quality of the tin coating can be obtained only via several (at least two) passages that need the tin coating to be applied at densities of 8 to 17 A/dm2 depending on the strip conveying speed in the line.
Moreover, the requirements on uniformity and corrosive resistance of coating become more rigid with decreasing the coating weight, which is not provided by the method under the above patent.
A problem found when using all the state of the art additives, such as Proxamin 385, is the control of foam that originates during recirculation of the electrolyte.
One of the aims of the present invention is to provide an electrolyte composition that permits to control such foam formation.
The nitrogen-containing block copolymers of ethylene and propylene oxides are known as addition agents in electrolytic deposition of tin (Pat. No RU2103418).
Such polymers are described in TU-6-36-0020 3335-95-94 FSUE “SRC NIOPIK”, Moscow.